Lighting device

ABSTRACT

A lighting device ( 1 ) comprising at least one light source ( 4 ) arranged in a housing ( 2 ) for emitting a lighting beam through a light-transmitting plate ( 3 ) of the housing ( 2 ), wherein said plate ( 3 ) is provided with means which reflect incident light on the plate ( 3 ), in such a manner that light which locally has a higher intensity is reflected more strongly at that location than light which locally has a lower intensity, with the special feature that said light-transmitting plate ( 3 ) and said means together form a constructional element made in one piece of a diffuse reflective material.

The invention relates to a lighting device comprising at least one lightsource arranged in a housing for emitting a lighting beam through alight-transmitting plate of the housing, wherein said plate is providedwith means which reflect incident light on the plate, in such a mannerthat light which locally has a higher intensity is reflected morestrongly at that location than light which locally has a lowerintensity.

Such a lighting device is generally known. The known lighting device isusually a flat light box, such as the light box that is used for thevisual inspection of x-ray photographs, for realising flat lightingtiles or lighting walls attached to walls or ceilings for generallighting purposes, or for backlighting advertising columns, billboards,or LCD screens. As a rule, the at least one light source that is presentin the light box is at least partly surrounded by a reflector forreflecting the light emitted by the light source in a direction awayfrom the light-transmitting plate back to the light-transmitting plate.An important practical requirement concerning such products is that thelight exiting from the light-transmitting plate must exhibit asubstantially homogeneous intensity over substantially the entire platearea, so that the location and the shape of the light source—for examplein the form of one or more TL tubes in the case of a light box—cannot bedistinguished as such from the outside. In order to accomplish this, itis known to apply a coating having a locally varying thickness, usuallyconsisting of light-diffusing inorganic particles dispersed in anorganic binder matrix, to the entire light-transmitting plate, forinstance by spraying. The locally varying thickness causes the opticalreflection of incident light on the plate to vary locally, wherein acoating which is locally relatively thicker effects a higher degree ofreflection at the location in question than a coating which is locallyrelatively thinner. Consequently, the local variation in thickness inthe coating must be selected so that the coating reflects incident lighton the plate in such a manner that light which locally has a higherintensity is reflected more strongly by the coating at the location inquestion than light which locally has a lower intensity.

One drawback of the known lighting device is the fact that, inparticular in the case of very flat light boxes, the required lateralthickness profile of the coating on the plate must exhibit a lateralthickness gradient which is so large and which, moreover, has beenadjusted so precisely in order to effect the desiredreflection/transmission gradient over the entire light-transmittingplate, that, in practice, it is not possible to apply such a coatingsufficiently reliably and efficiently.

The object of the invention is to overcome this drawback of the priorart, and in order to accomplish that objective a lighting device of thekind according to the invention as referred to in the introduction ischaracterized in that said light-transmitting plate and said meanstogether form a constructional element made in one piece of a diffusereflective material. In other words, said light-transmitting plate is adiffuse reflective plate of its own, hereinafter also referred to as adiffusor, by manufacturing the plate of a diffuse reflective material.The visual effect that is achieved therewith is that the light emittedby the lighting device has a laterally homogeneous intensity.

In one preferred embodiment of a lighting device in accordance with theinvention said element is made of a plastic material comprising diff-usereflective particles. Particularly, said diffuse reflective particlescomprise calcium halophosphate, calcium pyrophosphate, MgO, YBO₃, TiO₂or Al₂O₃ particles. In the alternative, said element of a plasticmaterial comprises air bubbles enclosed in the plastic material in orderto create a diffuse reflective material. Diffuse reflection can also berealized by a cristalline structure of said plastic material. Saidplastic material is preferably chosen from the group consisting ofacrylic plastics, fluoroplastics, polysiloxanes, polyesters,polycarbonates.

In another preferred embodiment of a lighting device according to theinvention said element comprises a profile with a varying thickness insuch a manner that the thickness of the element at a location close tothe light source is larger than at a location further removed from thelight source. Said profile may be located on a side of the diffusorfacing away or facing towards the light source. More in particular, thetransmission of light by the diffusor at a location opposite the lightsource amounts to 80% of the transmission of light by the diffusor at alocation being at a maximum distance from the light source.

In another preferred embodiment of a lighting device in accordance withthe invention said profile is made through grinding or embossing. In thealternative, said profile is made through moulding or extrusion.

The invention also refers to a method for laterally homogenising of theintensity of light emitted from a lighting device comprising at leastone light source arranged in a housing for emitting a lighting beamthrough a light-transmitting plate of the housing, wherein said plate isprovided with means which reflect incident light on the plate, in such amanner that light which locally has a higher intensity is reflected morestrongly at that location than light which locally has a lowerintensity, characterized in that said light-transmitting plate and saidmeans together are formed as a constructional element made in one pieceof a diffuse reflective material.

The invention will now be explained in more detail with reference to afigure illustrated in a drawing, which drawing is a schematic,cross-sectional view of a lighting device according to the invention.

The FIGURE shows a lighting device in the form of a light box 1including a reflective material 2 on its innerside and a glass orplastic plate 3 mounted thereon. In order to ensure that TL-tubes 4 inthe housing 2 cannot be distinguished from the outside, incident lighton the plate 3 coming from the TL-tubes 4 must be made to exit the plate3 with a homogeneous intensity over the entire area of the plate 3. Tothis end, the plate 3 is build as a diffusor of its own, that is ittransmits light from the TL-tubes in a diffuse reflective manner bymaking the plate 3 as such of a diffuse reflective material, for examplePMMA (PolyMethylMetAcrylate) having Al₂O₃ particles dispersed therein.

By making the plate 3 locally thicker at a location opposite theTL-tubes 4 than elswhere, light which locally has a higher intensitywill be reflected more strongly at that location (i.e. at a locationopposite the TL-tubes 4) than light which locally has a lower intensity(i.e. at a location between neighbouring TL-tubes 4, for example).

The invention is not retsricted to the embodiment shown, but extendsalso to other embodiments falling within the scope of the appendedclaims.

1. A lighting device comprising at least one light source arranged in ahousing for emitting a lighting beam through a light-transmitting plateof the housing, wherein said plate is provided with means which reflectincident light on the plate, in such a manner that light which locallyhas a higher intensity is reflected more strongly at that location thanlight which locally has a lower intensity, characterized in that saidlight-transmitting plate and said means together form a constructionalelement made in one piece of a diffuse reflective material.
 2. Alighting device according to claim 1, wherein said element is made of aplastic material comprising diffuse reflective particles.
 3. A lightingdevice according to claim 2, wherein said diffuse reflective particlescomprise calcium halophosphate, calcium pyrophosphate, MgO, YBO₃, TiO₂or Al₂O₃ particles.
 4. A lighting device according to claim 2, whereinsaid plastic material is chosen from the group consisting of acrylicplastics, fluoroplastics, polysiloxanes, polyesters, polycarbonates. 5.A lighting device according to claim 1, wherein said element comprises aprofile with a varying thickness in such a manner that the thickness ofthe element at a location close to the light source is larger than at alocation further removed from the light source.
 6. A lighting deviceaccording to claim 5, wherein said profile is made through grinding orembossing.
 7. A lighting device according to claim 5, wherein saidprofile is made through moulding or extrusion.
 8. A method for laterallyhomogenising of the intensity of light emitted from a lighting devicecomprising at least one light source arranged in a housing for emittinga lighting beam through a light-transmitting plate of the housing,wherein said plate is provided with means which reflect incident lighton the plate, in such a manner that light which locally has a higherintensity is reflected more strongly at that location than light whichlocally has a lower intensity, characterized in that saidlight-transmitting plate and said means together are formed as aconstructional element made in one piece of a diffuse reflectivematerial.